1. Field of the Invention
The present invention relates to an electrophotographic image forming apparatus which produces images using toner and which includes at least a photoreceptor and a cleaning device including a cleaning blade for cleaning the surface of the photoreceptor.
2. Discussion of the Related Art
Conventional electrophotographic image forming apparatus typically produce visual images by performing the following processes:
(1) Charging a rotated electrophotographic photoreceptor (hereinafter referred to as photoreceptor or image bearing member), which has a drum form or a belt form, with a charging device to uniformly charge the peripheral surface of the photoreceptor (charging process);(2) Irradiating the charged photoreceptor with imagewise light to form an electrostatic latent image thereon (light irradiating process);(3) Developing the electrostatic latent image with a developer including a toner to form a toner image on the photoreceptor (developing process);(4) Transferring the toner image onto a receiving material optionally via an intermediate transfer medium (transferring process);(5) Fixing the toner image on the receiving material, resulting in formation of a visual image (fixing process); and(6) Cleaning the surface of the photoreceptor with a cleaning device so that the photoreceptor is ready for the next image forming operation (cleaning process).
As for the photoreceptor, organic photoreceptors in which a photosensitive layer including an organic photosensitive material is located on a substrate are typically used. Among the organic photoreceptors, layered organic photoreceptors having a photosensitive layer in which a layer including a charge generation material and another layer including a charge transport material are overlaid are mainly used because of having advantages in cost, productivity and material selection flexibility (i.e., various materials can be used therefor).
In the above-mentioned image formation processes, the electrophotographic photoreceptor faces various hazards such as mechanical external forces, electric hazards and chemical hazards, thereby deteriorating the photoreceptor. Particularly, since color images are frequently produced recently, the photoreceptor is required to have good durability so that the photoreceptor can produce high quality images over a long period of time.
The photoreceptor used for such image forming apparatus faces the following mechanical hazards. Specifically, the photoreceptor is contacted and rubbed with toner which typically includes toner particles including a colorant and a binder resin, and a hard particulate inorganic material serving as an external additive. In addition, the photoreceptor is strongly pressed to a paper sheet, which serves as a receiving material and which typically includes a hard filler such as fibers and clays, in the transferring process. Further, the photoreceptor is strongly scraped with a cleaning blade in the cleaning process. Therefore, the surface of the photoreceptor tends to be abraded. In attempting to impart good abrasion resistance to the photoreceptor, highly durable resins such as polycarbonate and polyarylate are typically used as the binder resin of the photoreceptor as disclosed in published unexamined Japanese patent applications Nos. (hereinafter referred to as JP-As) 01-118139 and 10-73946.
Photoreceptors having a protective layer as the outermost layer have been proposed in attempting to impart good mechanical durability thereto. For example, protective layers in which a hard particulate metal oxide is dispersed, and crosslinked protective layers have been disclosed in JP-A 2001-166521. In addition, JP-As 63-65449 and 2002-196523 have disclosed techniques such that a good lubricating property is imparted to the surface of the photoreceptor to reduce abrasion of the surface of the photoreceptor.
Cleaning devices used for electrophotographic image forming apparatus typically use a cleaning blade made of an elastic material such as urethane rubbers as a cleaning member because such a cleaning blade has a simple structure and good cleanability. In the cleaning device, the tip (i.e., tip edge line) of the blade is contacted with the peripheral surface of the photoreceptor to scrape residual toner particles off the surface of the photoreceptor.
Recently, a need exists for an electrophotographic image forming apparatus capable of producing high quality images. In attempting to fulfill the need, the particle diameter of toner becomes smaller and smaller and the form of toner particles becomes more and more similar to the spherical form. Therefore, spherical toner prepared by a polymerization method has been mainly used for electrophotographic image forming apparatus recently.
However, when small and spherical toner is used, it becomes difficult to well remove residual toner particles from a photoreceptor using a cleaning blade, resulting in defective cleaning. This is because the residual toner particles present at the contact point of the cleaning blade with the photoreceptor have a rotational moment and therefore the toner particles easily pass through the gap between the cleaning blade and the photoreceptor. Therefore, when small and spherical toner is used, it is preferable to increase the pressure (hereinafter referred to as linear pressure) applied to the cleaning blade to prevent residual toner particles from passing through the gap between the cleaning blade and the photoreceptor.
When the linear pressure is increased, small and spherical toner can be well removed. However, increasing the linear pressure tends to cause problems in that the surface of the photoreceptor is seriously abraded; torque of driving the photoreceptor has to be increased; and the cleaning blade is seriously abraded.
Therefore, even when a photoreceptor having good mechanical durability is developed and used, there is a case where a small scratch or crack is formed on the surface of the photoreceptor after long repeated use or a case where the tip of the cleaning blade is chipped and thereby residual toner particles pass through the damaged portion, resulting in formation of a streak image (i.e., deterioration of image quality). This image quality deterioration problem is an urgent problem to be quickly solved for such a highly durable photoreceptor. In this regard, JP-A 2002-169315 discloses a technique in that a cleaning blade is pressed to a photoreceptor at a predetermined linear pressure, wherein the photoreceptor has a protective layer (outermost layer) including a crosslinked material.
Among various factors affecting the abrasion of photoreceptor, the most important factor is the pressure to the cleaning blade. If the transferability of toner can be improved, the pressure can be decreased, resulting in prevention of chipping of the cleaning blade. JP-A 2001-66814 discloses a technique in that projected and recessed portions having a special form such as prism forms, wave forms, cone forms, pyramid forms, and well forms are formed on the surface of a photoreceptor using a touch roll or stamper. In addition, it is described therein that by including a filler including silicon and fluorine in the entire charge transport layer to improve the transferability of toner and to reduce the stress applied to the photoreceptor. However, the technique has a drawback in that when the photoreceptor having projected and recessed portions thereon is repeatedly subjected to image forming operations and thereby the friction resistance of the photoreceptor surface is increased, the cleaning blade is easily chipped, thereby causing the streak image problem in that the residual toner particles pass through the chipped portion, resulting in formation of a streak image.
Because of these reasons, a need exists for an image forming apparatus having good cleanability, i.e., an image forming apparatus capable of producing high quality images without causing the streak image problem and toner filming problem, which are caused by passing of toner through the gap between the cleaning blade and the photoreceptor due to vibration, twisting, reversing and chipping of the cleaning blade.